Many products we use everyday are made from fibers. Examples of just a few of these products include paper, personal hygiene products, diapers, plates, containers, and packaging. Making products from wood fiber, fabric fiber and the like, involves breaking solid matter into fibrous matter. This also involves processing the fibrous matter into individual fibers that become fibrillated or frayed so they more tightly mesh with each other to form a finished fiber product that is desirably strong, tough, and resilient.
In fiber product manufacturing, refiners are devices used to process the fibrous matter, such as wood chips, fabric, and other types of pulp, into fibers and to further fibrillate existing fibers. The fibrous matter is transported in liquid stock to each refiner using a feed screw driven by a motor.
Each refiner has at least one pair of circular ridged refiner disks that face each other. During refining, fibrous matter in the stock to be refined is introduced into a gap between the disks that usually is quite small. Relative rotation between the disks during operation fibrillates or grinds fibers in the stock as the stock passes radially outwardly between the disks.
One example of a refiner that is a disk refiner is shown and disclosed in U.S. Pat. No. 5,425,508. However, many different kinds of refiners are in use today. For example, there are counterrotating refiners, double disk or twin refiners, and conical disk refiners. Conical disk refiners are often referred to in the industry as CD refiners.
During operation, many refiner parameters are monitored. Examples of parameters include the power of the motor coupled to a rotor carrying at least one refiner disk, the mass flow rate of the stock slurry being introduced into the refiner, the force with which opposed refiner disks are being forced together, the flow rate of dilution water being added in the refiner to the slurry, and the refiner gap.
It has always been a goal to monitor conditions in the refining zone between the pairs of opposed refining disks. However, this has always been a problem because the conditions in the refining zone are rather extreme making it rather difficult to accurately measure parameters in the refining zone, such as temperature and pressure.
Sensors have been used in the past to monitor parameters relating to refiner operation that include, for example, consistency, stock pressure, stock temperature, dilution flow water rate, refiner gap, the pressure or force urging one refiner disc toward the other, refiner energy use, and other parameters. Most of the sensors employed to measure these parameters were not located in the refining zone. As a result, while useful information was obtained to help make refiner control decisions, there was often a time lag that occurred from the time that changes actually occurred in the refining zone to when the sensor or sensors monitoring one or more of the parameters detected a change. This often lead to an operator of the refiner or an automatic refiner control system making a change to a refiner control parameter, such as refiner gap, dilution water flow rate, chip mass flow rate, refiner disc pressure or force, or refiner disc best because it may not have been truly based upon actual conditions in the refiner zone. As a result, refiner process control changes are typically infrequently made so as to permit operation of the refiner to converge or settle to a steady state operating condition. Often, this takes a great deal of time, typically hours, for it to be determined whether the change made by the operator of the automatic refiner control system had the desire effect. If it did not, it is possible that the quality of the resultant fiber product ultimately produced may not meet quality control standards. When this happens, the fiber product may have to be scrapped or sold at reduced cost. For example, where the fiber product is paper, this time lag can cause the fiber that is outputted by the refiner to have a lower quality than desired. This can cause paper made with the fiber to fail to meet quality control criteria for strength or some other parameter. When this happens, the paper may be scrapped by putting it into a beater so it can be reused to make other paper or it is sold at a reduced price as job lot. More recently, attempts have been made to locate sensors in close proximity to the refiner zone. For example, U.S. Pat. No. 6,502,774 discloses a plurality of spaced apart bores in the refining surface of a refiner disc. Temperature sensors are disposed in the bores such that the sensing element is located below the bottom of an adjacent groove of the disc in which it is disposed. While this sensor assembly is capable of outputting a temperature measurement, the measurement outputted may not accurately reflect the temperature of stock in the refining zone. First, since the sensing element is located below the bottom of an adjacent groove, it can measure the temperature of the material of the refiner disc that surrounds the sensor assembly. Since refiner discs are typically made of metal and possess a considerable amount of mass, the temperature of material often differs, sometimes quite significantly, from the temperature of stock in the refining zone. As a result, temperature response is quite slow and not indicative of the actual temperature of stock in the refining zone.
Such sensor arrangements have been used in the past, but have not been satisfactory because of the effects of thermal inertia caused by the surrounding mass of the refiner disc. Refiner control systems that receive temperature data from such sensors, are not as effective in controlling refiner operation because of this inherent time lag. Due in part to this, the performance of these control systems has been less than optimal, leaving a great deal of room for improvement.
The reliability and robustness of sensor assemblies has also been an issue because of the rather harsh conditions to which they are exposed in the refining zone. They are subjected to vibration, shock, temperature fluctuations, and pressure fluctuations that all can occur during refiner operation. Any one of these things can cause sensor failure or a significant degradation in sensor performance. Where a sensor is part of an array or group of sensors mounted to a refiner disc or in between refiner discs, the loss or degradation in performance of just a single sensor can have a significant impact. One known problem that exists for temperature sensors is that the sensing element holder can loosen over time and get pushed axially into the refining disc in which it is disposed. When this happens, the steam tight seal between the sensor assembly and the refiner disc can be compromised thereby causing steam and stock to leak from the refining zone through the bore in the refining surface completely through the disc. Such a leak can lower the pressure in the refining zone, which can reduce refining efficiency, quality, and throughput. Worse yet, stock and steam leaking from damaged sensor as well as other sensors that have not been damaged. This ultimately can lead to failure of the entire array or group of sensors, effectively rendering the refiner control system inoperative. Where leakage becomes too great, production will have to be stopped to change the sensor refiner disc. When such down time is unplanned, it is particularly costly.
What is needed is a more reliable and robust sensor assembly that is better able to withstand vibration, impact, shock, pressure fluctuations, and temperature fluctuations during refiner operation while still being able to provide a temperature measurement that is representative of a temperature of stock in the refining zone. What is also needed is a sensor refiner assembly that minimizes effects caused by leakage of stock and steam from the refining zone should a leak develop through one of the sensor assembly receiving bores in the refining surface of a sensor refiner disc.